Hot rolled ferritic stainless steel sheet, hot rolled and annealed ferritic stainless steel sheet and method for manufacturing the same

ABSTRACT

Provided are a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet and methods for manufacturing these steel sheets. A hot rolled ferritic stainless steel sheet having a chemical composition containing, by mass %, C: 0.005% to 0.060%, Si: 0.02% to 0.50%, Mn: 0.01% to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5% to 18.0%, Al: 0.001% to 0.10%, N: 0.005% to 0.100%, Ni: 0.1% to 1.0%, and the balance being Fe and inevitable impurities and an absolute value of planar anisotropy in terms of modulus of longitudinal elasticity below of 35 GPa or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/003286, filedJul. 11, 2016, which claims priority to Japanese Patent Application No.2015-142611, filed Jul. 17, 2015, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot rolled ferritic stainless steelsheet and a hot rolled and annealed ferritic stainless steel sheet whichhave sufficient corrosion resistance and excellent rigidity and methodsfor manufacturing these steel sheets.

BACKGROUND OF THE INVENTION

Nowadays, since regulations regarding automobile exhaust gas are beingstrengthened, improvement of fuel efficiency is an urgent task.Therefore, there is a trend toward using an exhaust gas recirculation(EGR) system in which exhaust gas discharged from an automobile engineis reused as the intake gas of the engine. The exhaust gas dischargedfrom an engine is passed through an EGR cooler, which is used forcooling the exhaust gas, and then charged again into the engine. Whenexhaust gas is recirculated, it is necessary to set a flange between therespective parts of the system in order to prevent the exhaust gas fromleaking. In particular, it is necessary that a flange which is used in aconnecting portion of a member such as an EGR cooler, which is alwayssubjected to vibration during running of an automobile, have sufficientrigidity in order to prevent gas from leaking from a gap which is formedbetween the parts due to the bending of the flange resulting from thevibration. Therefore, a flange having a large thickness (for example, athickness of 6 mm or more) is used as a flange which is fitted to amember such as an EGR cooler, which is always subjected to vibrationduring running of an automobile.

Conventionally, plain carbon steel is used for such a flange having alarge thickness. However, there is a risk of corrosion due to exhaustgas in the case of parts of an EGR system and the like through whichexhaust gas is passed. Therefore, consideration is being given to usingstainless steel which is superior to plain carbon steel in terms ofcorrosion resistance, and there is a demand for a hot rolled ferriticstainless steel sheet having a large thickness (for example, a thicknessof 6 mm or more) and sufficient rigidity to be used for a flange havinga large thickness.

For example, Patent Literature 1 discloses a hot rolled ferriticstainless steel sheet having a chemical composition containing, bymassa, C: 0.015% or less, Si: 0.01% to 0.4%, Mn: 0.01% to 0.8%, P: 0.04%or less, S: 0.01% or less, Cr: 14.0% to 18.0% (not inclusive), Ni: 0.05%to 1%, Nb: 0.3% to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10%or less, B: 0.0002% to 0.0020%, and the balance being Fe and inevitableimpurities, in which the contents of Nb, C, and N satisfy therelationship Nb/(C+N)≥16, a Charpy impact value at a temperature of 0°C. of 10 J/cm² or more, and a thickness of 5.0 mm to 9.0 mm.

In contrast, nowadays, there is a strong demand for relativelyinexpensive stainless steel (such as SUS 430 or 13Cr-stainless steel) inwhich the contents of chemical elements such as Ti and Nb, whichstabilize C and N, are as small as possible.

CITATION LIST Patent Literature

PTL 1: International Publication No. WO2014/157576

SUMMARY OF THE INVENTION

However, in the case where a conventional hot rolled ferritic stainlesssteel sheet, in which Ti or Nb is not contained, is formed into, forexample, the flange described above, there is a problem in that bendingand twist tend to occur, for example, when the flange is subjected tovibration.

An object of aspects of the present invention is, by solving theproblems described above, to provide a hot rolled ferritic stainlesssteel sheet and a hot rolled and annealed ferritic stainless steel sheetwhich have sufficient corrosion resistance and with which it is possibleto inhibit bending and twist from occurring after forming has beenperformed and methods for manufacturing these steel sheets.

The present inventors conducted close investigations in order to solvethe problems and, as a result, found that a steel sheet should havedecreased absolute value |ΔE| of planar anisotropy in terms of modulusof longitudinal elasticity, which is expressed by equation (1) below, inorder to inhibit deformation such as bending or twist when the steelsheet is used for, for example, a flange and then subjected tovibration. Moreover, it was found that it is possible to sufficientlyput the steel sheet into practical use for, for example, a flange in thecase where the absolute value of planar anisotropy in terms of modulusof longitudinal elasticity is 35 GPa or less.

|ΔE|=|(E _(L)−2×E _(D) +E _(C))/2|  (1)

Here, E_(L) denotes modulus of longitudinal elasticity (GPa) in adirection parallel to the rolling direction, E_(D) denotes modulus oflongitudinal elasticity (GPa) in a direction at an angle of 45° to therolling direction, and E_(C) denotes modulus of longitudinal elasticity(GPa) in a direction at a right angle to the rolling direction.

In addition, E_(L), E_(D), and E_(C) are respectively defined as thevalues of modulus of longitudinal elasticity in the rolling direction ofa steel sheet, in a direction at an angle of 45° to the rollingdirection, and in a direction at a right angle to the rolling directionwhich are measured at a temperature of 23° C. by using a transverseresonant technique prescribed in JIS Z 2280 (1993).

In addition, it was found that, it is possible to significantly decreasethe degree of planar anisotropy in terms of modulus of longitudinalelasticity by appropriately controlling the chemical composition offerritic stainless steel and, in particular, by appropriatelycontrolling the rolling temperature range and accumulated rollingreduction ratio (=100−(the final thickness/the thickness before rollingin the final 3 passes is performed)×100[%]) of the final 3 passes of afinish hot rolling process composed of multiple passes.

Aspects of the present invention have been completed on the basis of theknowledge described above, and the subject matter of aspects of thepresent invention is as follows.

[1] A hot rolled ferritic stainless steel sheet having a chemicalcomposition containing, by mass %, C: 0.005% to 0.060%, Si: 0.02% to0.50%, Mn: 0.01% to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5%to 18.0%, Al: 0.001% to 0.10%, N: 0.005% to 0.100%, Ni: 0.1% to 1.0%,and the balance being Fe and inevitable impurities and an absolute value|ΔE| of planar anisotropy in terms of modulus of longitudinal elasticitycalculated by using equation (1) below of 35 GPa or less.

|ΔE|=|(E _(L)−2×E _(D) +E _(C))/2|  (1)

Here, E_(L) denotes modulus of longitudinal elasticity (GPa) in adirection parallel to the rolling direction, E_(D) denotes modulus oflongitudinal elasticity (GPa) in a direction at an angle of 45° to therolling direction, and E_(C) denotes modulus of longitudinal elasticity(GPa) in a direction at a right angle to the rolling direction.

[2] The hot rolled ferritic stainless steel sheet according to item [1]above, the steel sheet having the chemical composition furthercontaining, by massa, one, two, or more selected from Cu: 0.1% to 1.0%,Mo: 0.1% to 0.5%, and Co: 0.01% to 0.5%.

[3] The hot rolled ferritic stainless steel sheet according to item [1]or [2] above, the steel sheet having the chemical composition furthercontaining, by mass %, one, two, or more selected from V: 0.01% to0.25%, Ti: 0.001% to 0.015%, Nb: 0.001% to 0.025%, Mg: 0.0002% to0.0050%, B: 0.0002% to 0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01%to 0.10%.

[4] A hot rolled and annealed ferritic stainless steel sheet obtained byperforming hot rolled sheet annealing on the hot rolled ferriticstainless steel sheet according to any one of items [1] to [3] above.

[5] A method for manufacturing the hot rolled ferritic stainless steelsheet according to any one of items [1] to [3] above, the methodincluding performing a hot rolling process involving finish rollingcomposed of 3 passes or more, in which rolling in the final 3 passes ofthe finish rolling is performed in a temperature range of 900° C. to1100° C. with an accumulated rolling reduction ratio of 25% or more.

[6] A method for manufacturing a hot rolled and annealed ferriticstainless steel sheet, the method including using the method formanufacturing a hot rolled ferritic stainless steel sheet according toitem [5] above, and further performing hot rolled sheet annealing at atemperature of 800° C. to 900° C. after the hot rolling process.

According to aspects of the present invention, it is possible to obtaina hot rolled ferritic stainless steel sheet and a hot rolled andannealed ferritic stainless steel sheet which have sufficient corrosionresistance and with which it is possible to inhibit bending and twistfrom occurring after forming has been performed.

Here, the term “sufficient corrosion resistance” in accordance withaspects of the present invention means a case where a rust area ratio(=the rust area/the total area of a steel sheet×100 [%]) is 25% or lessafter having performed 8 cycles of a salt spray cyclic corrosion testprescribed in JIS H 8502, where the unit cycle includes salt spraying(35° C., 5-mass %-NaCl, 2-hour spraying), drying (60° C., relativehumidity=40%, 4 hours), and wetting (50° C., relative humidity 95%, 2hours) in this order, on a steel sheet whose surface has been polishedby using #600 emery paper and whose end surfaces are sealed.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The hot rolled ferritic stainless steel sheet and the hot rolled andannealed ferritic stainless steel sheet according to aspects of thepresent invention have a chemical composition containing, by mass %, C:0.005% to 0.060%, Si: 0.02% to 0.50%, Mn: 0.01% to 1.00%, P: 0.04% orless, S: 0.01% or less, Cr: 15.5% to 18.0%, Al: 0.001% to 0.10%, N:0.005% to 0.100%, Ni: 0.1% to 1.0%, and the balance being Fe andinevitable impurities and an absolute value |ΔE| of planar anisotropy interms of modulus of longitudinal elasticity calculated by using equation(1) below of 35 GPa or less.

|ΔE|=|(E _(L)−2×E _(D) +E _(C))/2|  (1)

Here, E_(L) denotes modulus of longitudinal elasticity (GPa) in adirection parallel to the rolling direction, E_(D) denotes modulus oflongitudinal elasticity (GPa) in a direction at an angle of 45° to therolling direction, and E_(C) denotes modulus of longitudinal elasticity(GPa) in a direction at a right angle to the rolling direction.

In addition, E_(L), E_(D), and E_(C) are respectively defined as thevalues of modulus of longitudinal elasticity in the rolling direction ofa steel sheet, in a direction at an angle of 45° to the rollingdirection, and in a direction at a right angle to the rolling directionwhich are measured at a temperature of 23° C. by using a transverseresonant technique prescribed in JIS Z 2280 (1993).

Hereafter, aspects of the present invention will be described in detail.

The hot rolled ferritic stainless steel sheet and hot rolled andannealed ferritic stainless steel sheet according to aspects of thepresent invention are intended to be used mainly for a flange having alarge wall thickness which is used for the EGR cooler parts of anautomobile. The present inventors used various kinds of hot rolledferritic stainless steel sheet for a flange having a large thickness foran EGR cooler in order to evaluate its performance in detail. As aresult, it was found that large bending and twist tend to occur due tovibration during running of an automobile in the case where a hot rolledferritic stainless steel sheet having an absolute value of planaranisotropy in terms of the modulus of longitudinal elasticity of morethan 35 GPa is used.

Therefore, the present inventors diligently conducted investigationsregarding a method for decreasing the degree of planar anisotropy interms of the modulus of longitudinal elasticity of a hot rolled ferriticstainless steel sheet, in particular, focusing on the rollingtemperature and rolling reduction ratio of each of multiple passes of ahot rolling process using multiple rolling stands. As a result, it wasfound that it is possible to significantly decrease the degree of planaranisotropy in terms of modulus of longitudinal elasticity and to achievethe desired rigidity by performing rolling in the final 3 passes ofmultiple-pass finish hot rolling composed of 3 passes or more in atemperature range of 900° C. to 1100° C. with an accumulated rollingreduction ratio of 25% or more (or preferably 30% or more).

The reasons why the desired degree of planar anisotropy in terms ofmodulus of longitudinal elasticity is achieved by using the methoddescribed above will be described hereafter.

The modulus of longitudinal elasticity of a hot rolled ferriticstainless steel sheet strongly depends on the texture of the steelsheet. Since the texture of a hot rolled steel sheet is formed byrepeating the application of processing strain due to rolling andrecrystallization, it is possible to control such a texture by adjustingtemperature at which rolling work is applied and the amount of strainapplied due to rolling.

On the other hand, in the central portion in the thickness direction ofa ferritic stainless steel slab which has not yet been subjected to hotrolling, elongated ferrite grains are sequentially distributed in thecasting direction. In the case where such a stainless steel slab issubjected to hot rolling by using a conventional method, since there isa decrease in grain boundary area due to an increase in the number ofelongated grains in the central portion in the thickness direction, thenumber of recrystallization sites is less in the central portion in thethickness direction than in the surface layer of the steel sheet.

Moreover, in the case where a steel sheet is rolled, the steel sheet iselongated with deformation starting mainly in the surface layer.Therefore, in the case of a small rolling reduction ratio, the amount ofdeformation is small in the central portion in the thickness direction,which results in almost no rolling strain being introduced to thecentral portion in the thickness direction.

Then, in the case of conventional hot rolling, while the application ofstrain and recrystallization are repeated in the surface layer of asteel sheet, the progress of recrystallization is significantly delayedin the central portion in the thickness direction. Therefore, elongatedferrite grains having similar crystal orientations, which have beenformed in a casting process, tend to be retained without being broken,which results in an increase in the degree of planar anisotropy in termsof modulus of longitudinal elasticity after hot rolling has beenperformed.

As an optimum method for decreasing the degree of planar anisotropy interms of modulus of longitudinal elasticity, the present inventorsdevised a method in which rolling in the final 3 passes of finish hotrolling is performed in a temperature range of 900° C. to 1100° C. inwhich recrystallization actively occurs, with an accumulated rollingreduction ratio of 25% or more, which is a rolling reduction larger thanthat in conventional art.

Specifically, the present inventors systematically conductedinvestigations regarding the influences of temperature and rollingreduction ratio at which each rolling pass of finish hot rollingcomposed of 7 passes was performed on the degree of planar anisotropy interms of the modulus of longitudinal elasticity of a hot rolled steelsheet manufactured. As a result, it was found that there is a tendencyfor the degree of planar anisotropy in terms of the modulus oflongitudinal elasticity of the steel sheet after hot rolling has beenperformed to strongly depend on the rolling temperatures and rollingreduction ratios of the final 3 passes while there is almost noinfluence of the temperatures and rolling reduction ratios of the first4 passes. Therefore, the present inventors conducted closerinvestigations regarding the influences of the rolling temperatures androlling reduction ratios of the final 3 passes and the accumulatedrolling reduction ratio of the final 3 passes: As a result, it was foundthat there is a tendency for the degree of planar anisotropy in terms ofthe modulus of longitudinal elasticity of a hot rolled steel sheet tosignificantly decrease in the case where rolling in the final 3 passesis performed in a temperature range of 900° C. to 1100° C. and that theamount of change in the degree of planar anisotropy in terms of themodulus of longitudinal elasticity of the hot rolled steel sheet in thiscase depends not on the rolling reduction ratio of each of the passesbut on the accumulated rolling reduction ratio of the final 3 passes.That is, it was found that it is important to complete finish rolling byperforming rolling in a temperature range of 900° C. to 1100° C. with anaccumulated rolling reduction ratio of 25% or more from the viewpoint ofthe planar anisotropy in terms of the modulus of longitudinal elasticityof a hot rolled steel sheet.

The present inventors conducted investigations regarding the reasons whythe rolling temperature and rolling reduction ratio of each of therolling passes prior to the final 3 passes have a small influence on theplanar anisotropy in terms of the modulus of longitudinal elasticity ofa hot rolled steel sheet. As a result, it was found that, in the case ofthe rolling passes prior to the final 3 passes, since the thicknessbefore rolling is performed is large, sufficient rolling strain is notapplied to the central portion in the thickness direction even if therolling reduction ratio is large. In addition, it was found that, sincerolling temperature is high, there is an increase in grain size due toan excessive growth of recrystallized crystal grains which are formedafter rolling has been performed, which results in a significantly smalleffect of decreasing the degree of anisotropy in a metallographicstructure through the forming of the recrystallized crystal grainscompared with the accumulated effect in the case of the final 3 passes.

On the other hand, in the case where the accumulated rolling ratio ofthe final 3 passes is 25% or more, which is larger than that inconventional art, since rolling strain is effectively applied to thecentral portion in the thickness direction of a steel sheet due torolling being performed in the final 3 passes, there is a significantincrease in the number of recrystallization sites in the central portionin the thickness direction. By performing such rolling in a temperaturerange of 900° C. to 1100° C., in which recrystallization activelyoccurs, since recrystallization is promoted in the central portion inthe thickness direction, an elongated-ferrite-grain structure, which wasformed in a casting process, is effectively broken, which results in asignificant decrease in the degree of planar anisotropy in terms ofmodulus of longitudinal elasticity after hot rolling has been performed.In addition, by performing rolling at a temperature of 1100° C. orlower, since it is possible to inhibit an increase in the grain size ofrecrystallized crystal grains, the effect of decreasing the degree ofanisotropy in a metallographic structure is sufficiently realized. Withthis technique, since it is possible to control the absolute value ofplanar anisotropy in terms of modulus of longitudinal elasticity to be35 GPa or less, it is possible to inhibit deformation such as largebending and twist when a steel sheet is subjected to vibration after thesteel sheet has been formed into, for example, a flange having a largewall thickness.

Moreover, the present inventors found that, in the case where the hotrolled steel sheet according to aspects of the present invention issubjected to hot rolled sheet annealing in a temperature range of 800°C. to 900° C. in order to improve the formability of the hot rolledsteel sheet, the effect of decreasing the degree of planar anisotropy interms of modulus of longitudinal elasticity, which have been obtainedthrough hot rolling, is maintained while the effect of improvingformability of the hot rolled and annealed steel sheet is obtained. Itwas found that this is because the effect of decreasing the degree ofplanar anisotropy in terms of modulus of longitudinal elasticity inaccordance with aspects of the present invention is caused by thebreakage of an elongated-ferrite-grain structure in the central portionin the thickness direction and because elongated ferrite grains, whichcontribute to an increase in the degree of anisotropy in a steel sheet,are not formed in the case where hot rolled sheet annealing is performedin a specified temperature range after hot rolling has been performed.

In addition, although there is no particular limitation on the thicknessof the hot rolled ferritic stainless steel sheet and the hot rolled andannealed ferritic stainless steel sheet according to aspects of thepresent invention, it is preferable that the thickness be 5.0 mm to 15.0mm, because the steel sheet desirably has a thickness suitable for aflange having a large wall thickness.

Hereafter, the chemical composition of the ferritic stainless steelsheet and the hot rolled and annealed ferritic stainless steel sheetaccording to aspects of the present invention will be described.

Hereafter, % used when describing a chemical composition means mass %,unless otherwise noted.

C: 0.005% to 0.060%

In the case where the C content is large, there is a deterioration inworkability, and there is sensitization and a deterioration in toughnessdue to the precipitation of Cr-based carbonitrides. Therefore, the upperlimit of the C content is set to be 0.060%. On the other hand, there isa significant increase in refining costs in the case where the C contentis excessively small. Therefore, the lower limit of the C content is setto be 0.005%, which is at a level at which there is no significantincrease in manufacturing costs in a common refining method. It ispreferable that the C content be 0.010% to 0.050% from the viewpoint ofthe stable manufacturability in a steel-making process. The C content ismore preferably in a range of 0.020% to 0.045%, even more preferably0.025% to 0.040%, or even much more preferably 0.030% to 0.040%.

Si: 0.02% to 0.50%

Si is a chemical element which functions as a deoxidizing agent in aprocess for preparing molten steel. It is necessary that the Si contentbe 0.02% or more in order to obtain such an effect. However, it is notdesirable that the Si content be more than 0.50%, because this resultsin a deterioration in manufacturability in a hot rolling process due toan increase in rolling load when hot rolling is performed as a result ofan increase in the hardness of a steel sheet. Therefore, the Si contentis set to be in a range of 0.02% to 0.50%, preferably 0.10% to 0.35%, ormore preferably 0.10% to 0.30%.

Mn: 0.01% to 1.00%

It is not desirable that the Mn content be excessively large, becausethis results in a deterioration in manufacturability in a hot rollingprocess due to an increase in rolling load when hot rolling is performedas a result of an increase in the hardness of a steel sheet as in thecase of Si. In addition, there may be a deterioration in corrosionresistance due to an increase in the amount of MnS. Therefore, the upperlimit of the Mn content is set to be 1.00%. The lower limit of the Mncontent is set to be 0.01% from the viewpoint of a load placed on arefining process. It is preferable that the Mn content be in a range of0.10% to 0.90%, or more preferably 0.45% to 0.85%.

P: 0.04% or less

Since P is a chemical element which promotes intergranular fracture dueto intergranular segregation, it is desirable that the P content be assmall as possible, and the upper limit of the P content is set to be0.04%. It is preferable that the P content be 0.03% or less, or morepreferably 0.01% or less.

S: 0.01% or less

S is a chemical element which deteriorates, for example, ductility andcorrosion resistance as a result of existing in the form ofsulfide-based inclusions such as MnS, and such negative effects becomemarked, in particular, in the case where the S content is more than0.01%. Therefore, it is desirable that the S content be as small aspossible, and the upper limit of the S content is set to be 0.01% inaccordance with aspects of the present invention. It is preferable thatthe S content be 0.007% or less, or more preferably 0.005% or less.

Cr: 15.5% to 18.0%

Cr is a chemical element which is effective for improving corrosionresistance by forming a passivation film on the surface of a steelsheet. It is necessary that the Cr content be 15.5% or more in order toobtain such an effect. However, it is not desirable that the Cr contentbe more than 18.0%, because this results in a significant deteriorationin the toughness of a steel sheet. Therefore, the Cr content is set tobe in a range of 15.5% to 18.0%, preferably 16.0% to 17.0%, or morepreferably 16.0% to 16.5%.

Al: 0.001% to 0.10%

Al is, like Si, a chemical element which functions as a deoxidizingagent. It is necessary that the Al content be 0.001% or more in order toobtain such an effect. However, in the case where the Al content is morethan 0.10%, since there is an increase in the amount of Al-basedinclusions such as Al₂O₃, there is a tendency for surface quality todeteriorate. Therefore, the Al content is set to be in a range of 0.001%to 0.10%, preferably 0.001% to 0.07%, or more preferably 0.001% to0.05%.

N: 0.005% to 0.100%

In the case where the N content is large, as in the case of C, there isa deterioration in workability, and there is sensitization and adeterioration in toughness due to the precipitation of Cr-basedcarbonitrides. Therefore, the upper limit of the N content is set to be0.100%. On the other hand, there is a significant increase in refiningcosts in the case where the N content is excessively small as in thecase of C. Therefore, the lower limit of the N content is set to be0.005%, which is at a level at which there is no significant increase inmanufacturing costs in a common refining method. It is preferable thatthe N content be 0.010% to 0.075% from the viewpoint of stablemanufacturability in a steel-making process. The N content is morepreferably in a range of 0.025% to 0.055%, or even more preferably0.030% to 0.050%.

Ni: 0.1% to 1.0%

Ni is a chemical element which improves corrosion resistance, and theaddition of Ni is effective, in particular, in the case where highcorrosion resistance is required. Such an effect becomes marked in thecase where the Ni content is 0.1% or more. However, it is not desirablethat the Ni content be more than 1.0%, because this results in adeterioration in formability. Therefore, the Ni content is set to be0.1% to 1.0%. The Ni content is preferably in a range of 0.2% to 0.4%.

The remainder is Fe and inevitable impurities.

Although it is possible to obtain the effects of aspects of the presentinvention by using the chemical composition described above, thechemical composition may further contain the following chemical elementsin order to improve manufacturability or material properties.

One, two, or more selected from Cu: 0.1% to 1.0%, Mo: 0.1% to 0.5%, andCo: 0.01% to 0.5%

Cu: 0.1% to 1.0%

Cu is a chemical element which improves corrosion resistance, and theaddition of Cu is effective, in particular, in the case where highcorrosion resistance is required. Such an effect becomes marked in thecase where the Cu content is 0.1% or more. However, in the case wherethe Cu content is more than 1.0%, there may be a deterioration informability. Therefore, in the case where Cu is added, the Cu content isset to be 0.1% to 1.0%. The Cu content is preferably in a range of 0.2%to 0.4%.

Mo: 0.1% to 0.5%

Mo is, like Ni and Cu, a chemical element which improves corrosionresistance, and the addition of Mo is effective, in particular, in thecase where high corrosion resistance is required. Such an effect becomesmarked in the case where the Mo content is 0.1% or more. However, in thecase where the Mo content is more than 0.5%, there may be adeterioration in manufacturability in a hot rolling process due to anincrease in rolling load when hot rolling is performed as a result of anincrease in the hardness of a steel sheet. Therefore, in the case whereMo is added, the Mo content is set to be 0.1% to 0.5%. The Mo content ispreferably in a range of 0.2% to 0.3%.

Co: 0.01% to 0.5%

Co is a chemical element which improves toughness. Such an effect isobtained in the case where the Co content is 0.01% or more. On the otherhand, in the case where the Co content is more than 0.5%, there may be adeterioration in formability. Therefore, in the case where Co is added,the Co content is set to be in a range of 0.01% to 0.5%.

One, two, or more selected from V: −0.0l % to 0.25%, Ti: 0.001% to0.015%, Nb: 0.001% to 0.025%, Mg: 0.0002% to 0.0050%, B: 0.0002% to0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%

V: 0.01% to 0.25%

V is a chemical element which forms carbonitrides more readily than Cr.V is effective for inhibiting sensitization, which is caused by theprecipitation of Cr carbonitrides, by precipitating C and N in steel inthe form of V-based carbonitrides when hot rolling is performed. It isnecessary that the V content be 0.01% or more in order to obtain such aneffect. However, in the case where the V content is more than 0.25%,there may be deterioration in workability, and there is an increase inmanufacturing costs. Therefore, in the case where V is added, the Vcontent is set to be in a range of 0.01% to 0.25%, or preferably 0.03%to 0.08%.

Ti: 0.001% to 0.015% and Nb: 0.001% to 0.025%

Ti and Nb are, like V, chemical elements which have a high affinity forC and N and which are effective for inhibiting sensitization, which iscaused by the precipitation of Cr carbonitrides, by precipitating in theform of carbides and nitrides when hot rolling is performed. In order toobtain such an effect, it is necessary that the Ti content be 0.001% ormore or that the Nb content be 0.001% or more. However, in the casewhere the Ti content is more than 0.015% or in the case where the Nbcontent is more than 0.030%, there may be a case where it is notpossible to achieve good surface quality due to the precipitation of anexcessive amount of TiN or NbC. Therefore, the Ti content is set to bein a range of 0.001% to 0.015% in the case where Ti is added, and the Nbcontent is set to be in a range of 0.001% to 0.025% in the case where Nbis added. It is preferable that the Ti content be in a range of 0.003%to 0.010%. It is preferable that Nb content be in a range of 0.005% to0.020%, or more preferably 0.010% to 0.015%.

Mg: 0.0002% to 0.0050%

Mg is a chemical element which is effective for improving hotworkability. It is necessary that the Mg content be 0.0002% or more inorder to obtain such an effect. However, in the case where the Mgcontent is more than 0.0050%, there may be a deterioration in surfacequality. Therefore, in the case where Mg is added, the Mg content is setto be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%,or more preferably 0.0005% to 0.0020%.

B: 0.0002% to 0.0050%

B is a chemical element which is effective for preventing secondary coldwork embrittlement. It is necessary that the B content be 0.0002% ormore in order to obtain such an effect. However, in the case where the Bcontent is more than 0.0050%, there may be a deterioration in hotworkability. Therefore, in the case where B is added, the B content isset to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to0.0035%, or more preferably 0.0005% to 0.0020%.

Ca: 0.0002% to 0.0020%

Ca is a chemical element which is effective for preventing nozzleclogging due to the precipitation of inclusions which tends to occurwhen continuous casting is performed. It is necessary that the Cacontent be 0.0002% or more in order to obtain such an effect. However,in the case where the Ca content is more than 0.0020%, there may be adeterioration in corrosion resistance due to the formation of CaS.Therefore, in the case where Ca is added, the Ca content is set to be ina range of 0.0002% to 0.0020%, preferably 0.0005% to 0.0015%, or morepreferably 0.0005% to 0.0010%.

REM: 0.01% to 0.10%

REM (rare earth metals) is a chemical element which improves oxidationresistance and which is effective for improving the corrosion resistanceof, in particular, a weld zone by inhibiting the formation of an oxidefilm in the weld zone. It is necessary that the REM content be 0.01% ormore in order to obtain such an effect. However, in the case where theREM content is more than 0.10%, there may be a deterioration inmanufacturability such as pickling capability when cold-rolled sheetannealing is performed. In addition, since REM is an expensive chemicalelement, it is not preferable that the REM content be excessively large,because this results in an increase in manufacturing costs. Therefore,in the case where REM is added, the REM content is set to be in a rangeof 0.01% to 0.10%, or preferably 0.01% to 0.05%.

Hereafter, the method for manufacturing the ferritic stainless steelsheet and the hot rolled and annealed ferritic stainless steel sheetaccording to aspects of the present invention will be described.

It is possible to obtain the ferritic stainless steel sheet according toaspects of the present invention by performing a hot rolling processinvolving rough rolling and finish rolling composed of 3 passes or moreon a steel slab having the chemical composition described above, inwhich rolling in the final 3 passes of finish rolling is performed in atemperature range of 900° C. to 1100° C. with an accumulated rollingreduction ratio of 25% or more.

Here, there is no particular limitation on the maximum number of passesof finish rolling from the viewpoint of achieving the specified materialproperties. However, in the case where the maximum number of passes ismore than 15, since there is a tendency for the temperature of a steelsheet to decrease due to an increase in the number of contacts betweenthe sheet and rolling rolls, there may be a deterioration inmanufacturability or an increase in manufacturing costs, because, forexample, it is necessary to heat the steel sheet from outside in orderto maintain the temperature of the steel sheet within the specifiedtemperature range. Therefore, it is preferable that the maximum numberof passes be 15 or less, or more preferably 10 or less.

First, molten steel having the chemical composition described above isprepared by using a known method such as one which utilizes, forexample, a converter, an electric furnace, or a vacuum melting furnaceand made into a steel raw material (slab) by using a continuous castingmethod or an ingot casting-slabbing method.

This slab is subjected to hot rolling after having been heated at atemperature of 1100° C. to 1250° C. for 1 hour to 24 hours or the slabas cast is directly subjected to hot rolling without having been heated.In accordance with aspects of the present invention, although there isno particular limitation on rough rolling, it is preferable that anaccumulated rolling reduction ratio in rough rolling be 65% or more inorder to effectively break a cast structure. When finish rolling issubsequently performed in order to obtain a specified thickness, rollingin the final 3 passes of finish rolling is performed in a temperaturerange of 900° C. to 1100° C. with an accumulated rolling reduction ratioof 25% or more.

Rolling temperature range of final 3 passes: 900° C. to 1100° C.

In the final 3 passes of finish rolling, it is necessary to effectivelyapply rolling strain to the central portion in the thickness directionby a large accumulated rolling reduction ratio and to allow sufficientrecrystallization to occur. Therefore, it is necessary that rolling inthe final 3 passes of finish rolling be performed in a temperature rangeof 900° C. to 1100° C., in which sufficient recrystallization occurs. Inthe case where the rolling temperature of the final 3 passes is lowerthan 900° C., since there is insufficient recrystallization, it is notpossible to achieve the desired degree of planar anisotropy in terms ofmodulus of longitudinal elasticity. On the other hand, it is notpreferable that the rolling temperature of the final 3 passes be higherthan 1100° C., because this causes a significant increase in crystalgrain size with the result that it is not possible to achieve thespecified degree of planar anisotropy in terms of modulus oflongitudinal elasticity and with the result that the toughness of a hotrolled steel sheet is deteriorated.

It is preferable that the rolling temperature of the final 3 passes bein a range of 900° C. to 1075° C., or more preferably 930° C. to 1050°C. In addition, in order to prevent an excessive rolling load from beingplaced in one of the final 3 passes, it is preferable that rolling inthe first pass of the final 3 passes be performed in a temperature rangeof 950° C. to 1100° C., that rolling in the second pass following thefirst pass be performed in a temperature range of 925° C. to 1075° C.,and that rolling in the third pass following the second pass beperformed in a temperature range of 900° C. to 1050° C.

Accumulated rolling reduction ratio of final 3 passes: 25% or more

In order to effectively apply rolling strain to the central portion inthe thickness direction of a steel sheet, it is necessary that rollingin the final 3 passes of finish rolling be performed with an accumulatedrolling reduction ratio of 25% or more. In the case where theaccumulated rolling reduction ratio is less than 25%, sincerecrystallization in the central portion in the thickness direction isdelayed due to an insufficient amount of rolling strain applied to thecentral portion in the thickness direction, it is not possible toachieve the desired degree of planar anisotropy in terms of modulus oflongitudinal elasticity. Therefore, it is preferable that theaccumulated rolling reduction ratio be 25% or more, more preferably 30%or more, or even more preferably 35% or more. Here, although there is noparticular limitation on the upper limit of the accumulated rollingreduction ratio, in the case where the accumulated rolling reductionratio is excessively large, there is a deterioration inmanufacturability due to an increase in rolling load, and there may be acase where rough surface is caused after rolling has been performed.Therefore, it is preferable that the accumulated rolling reduction ratiobe 60% or less.

In addition, the accumulated rolling reduction ratio described above isexpressed by the formula 100−(the final thickness/the thickness beforerolling in the final 3 passes is performed)×100 [%].

In addition, the method for manufacturing the hot rolled ferriticstainless steel sheet according to aspects of the present invention ischaracterized in that the rolling temperature and accumulated rollingreduction ratio of the final 3 passes of finish rolling are controlled.In the case where the control target is the rolling temperature andaccumulated rolling reduction ratio of the final 4 passes or more, sincethe rolling reduction ratio of each of the passes is too small forapplied strain to contribute a decrease in the degree of planaranisotropy in terms of modulus of longitudinal elasticity, it is notpossible to sufficiently obtain the effect of decreasing the degree ofplanar anisotropy in terms of modulus of longitudinal elasticity. Inaddition, it is not preferable that the control target be the rollingtemperature and accumulated rolling reduction ratio of the final 2passes or less, because this may result in a deterioration inmanufacturability due to a significant increase in rolling load as aresult of performing high rolling reduction with an accumulated rollingreduction ratio of 25% or more in 2 passes. Therefore, in the method formanufacturing the hot rolled ferritic stainless steel sheet according toaspects of the present invention, the rolling temperature andaccumulated rolling reduction ratio of the final 3 passes of finishrolling are controlled.

In addition, in the method for manufacturing the hot rolled ferriticstainless steel sheet according to aspects of the present invention,there is no particular limitation on the number of passes of finishrolling as long as the number is 3 or more so that the rollingtemperature and accumulated rolling reduction ratio of the final 3passes of finish rolling are controlled.

After finish rolling has been performed, the steel sheet is cooled andthen subjected to a coiling treatment in order to obtain a hot rolledsteel strip. In accordance with aspects of the present invention,although there is no particular limitation on the coiling temperature,in the case where steel having a chemical composition with which anaustenite phase is formed during hot rolling is coiled at a coilingtemperature of lower than 500° C., since an austenite phase transformsinto a martensite phase, there may be a deterioration in formability dueto an increase in the hardness of a hot rolled steel sheet. Therefore,it is preferable that a coiling treatment be performed at a temperatureof 500° C. or higher.

In accordance with aspects of the present invention, it is possible toachieve the desired corrosion resistance and the desired degree ofplanar anisotropy in terms of modulus of longitudinal elasticity at thetime of the completion of the hot rolling described above, and further ahot rolled and annealed ferritic stainless steel sheet may bemanufactured by performing hot rolled sheet annealing on the hot rolledferritic stainless steel sheet in a temperature range of 800° C. to 900°C. after the hot rolling process in order to improve formability.

Hot rolled sheet annealing temperature: 800° C. to 900° C. In the casewhere the hot rolled sheet annealing temperature is lower than 800° C.,since there is insufficient recrystallization, it is not possible toobtain the effect of improving formability due to deformationmicrostructure formed by performing hot rolling being retained. On theother hand, in the case where the hot rolled sheet annealing temperatureis higher than 900° C., since there is an increase in the degree ofplanar anisotropy in terms of modulus of longitudinal elasticity due tothe formation of an austenite phase when annealing is performed, namely,there may be a case where the specified degree of planar anisotropy interms of modulus of longitudinal elasticity which has been obtained inthe hot rolled steel sheet is lost. In addition, in the case where acooling rate after hot rolled sheet annealing has been performed at atemperature of higher than 900° C. is large, since there is an increasein the hardness of a steel sheet due to an austenite phase transforminginto a martensite phase, there may be conversely a deterioration informability. Therefore, in the case where hot rolled sheet annealing isperformed, it is preferable that the annealing temperature be 800° C. to900° C. Here, there is no particular limitation on the holding time andmethod of hot rolled sheet annealing, any one of a box annealing (batchannealing) method and a continuous annealing method may be used.

The obtained hot rolled steel sheet or steel sheet (hot rolled andannealed steel sheet) which has been subjected to hot rolled sheetannealing may be subjected to a descaling treatment such as one whichutilizes shot blasting or pickling as needed. Moreover, grinding orpolishing may be performed in order to improve surface quality.

EXAMPLES

Hereafter, aspects of the present invention will be described in detailby using examples.

Molten stainless steels having the chemical compositions given in Table1 were prepared by performing refining which utilized a converter havinga capacity of 150 tons and a strong stirring-vacuum oxygendecarburization (SS-VOD) method, and steel slabs having a width of 1000mm and a thickness of 200 mm were then manufactured by using acontinuous casting method. The obtained slabs were heated at atemperature of 1200° C. for one hour and then subjected to hot rollingin which reverse-type rough rolling was performed by using 3 rollingstands in order to obtain steel sheets having a thickness of about 40 mmand in which the final 3 passes (the fifth pass, the sixth path, and theseventh pass) of finish rolling composed of 7 passes were then performedunder the conditions given in Table 2 in order to obtain hot rolledsteel sheets. In addition, some of the hot rolled steel sheets (Nos. 25,26, and 38 in Table 2) were subjected to hot rolled sheet annealing inwhich the hot rolled steel sheets were held under the conditions givenin Table 2 for 8 hours after hot rolling had been performed and in whichthe held steel sheets were subjected furnace cooling in order to obtainhot rolled and annealed steel sheets.

The obtained hot rolled steel sheets and hot rolled and annealed steelsheets were evaluated as described below.

(1) Evaluation of Planar Anisotropy

Test pieces having a length of 60 mm, a width of 10 mm, and a thicknessof 2 mm whose longitudinal direction were respectively a directionparallel to the rolling direction, a direction at an angle of 45° to therolling direction, and a direction at a right angle to the rollingdirection were taken from the central portion in the thickness directionwithin 1 mm on both sides of the center in the thickness direction. Themodulus of longitudinal elasticity of each of the obtained test pieceswas measured at a temperature of 23° C. by using a transverse resonanttechnique prescribed in JIS Z 2280 (1993), and the absolute value |ΔE|of planar anisotropy in terms of modulus of longitudinal elasticity wascalculated by using equation (1) below.

|ΔE|=|(E _(L)−2×E _(D) =E _(C))/2|  (1)

Here, E_(L) denotes modulus of longitudinal elasticity (GPa) in adirection parallel to the rolling direction, E_(D) denotes modulus oflongitudinal elasticity (GPa) in a direction at an angle of 45° to therolling direction, and E_(C) denotes modulus of longitudinal elasticity(GPa) in a direction at a right angle to the rolling direction.

A case where the degree of planar anisotropy in terms of modulus oflongitudinal elasticity, that is, |ΔE| was 35 GPa or less was judged asa case where it is possible to sufficiently inhibit bending and twistafter the steel sheet has been formed into, for example, a flange, thatis, judged as satisfactory (◯). A case where the degree of planaranisotropy in terms of modulus of longitudinal elasticity, that is, |ΔE|was more than 35 GPa was judged as unsatisfactory (x).

(2) Evaluation of Corrosion Resistance

A salt spray cyclic corrosion test prescribed in JIS H 8502 wasperformed on a test piece having a size of 60 mm×100 mm which had beentaken from the hot rolled steel sheet, whose surface had been polishedby using #600 emery paper, and whose end surfaces were sealed. The saltspray cyclic corrosion test was performed in such a manner that a unitcycle was repeated 8 times, where the unit cycle includes salt spraying(5-mass %-NaCl, 35° C., 2-hour spraying), drying (60° C., 4 hours,relative humidity=40%), and wetting (50° C., 2 hours, relativehumidity≥95%).

After having determined the rust area on the surface of the test pieceby performing image analysis on a photograph of the surface of the testpiece which had been obtained after the 8 cycles of the salt spraycyclic corrosion test, a rust area ratio ((the rust area of the testpiece/the total area of the test piece)×100 [%]) was calculated as theratio of the rust area to the total area of the test piece. A case wherethe rust area ratio was 10% or less was judged as a case of particularlyexcellent corrosion resistance, that is, judged as satisfactory (⊙), acase where the rust area ratio was more than 10% and 25% or less wasjudged as satisfactory (◯), and a case where the rust area ratio wasmore than 25% was judged as unsatisfactory (x).

The evaluation results are given in Table 2 along with the hot rollingconditions.

TABLE 1 Steel Chemical Composition (mass %) Code C Si Mn P S Cr Al N NiOther Note A 0.043 0.24 0.65 0.03 0.004 16.2 0.002 0.047 0.15 Example B0.016 0.16 0.81 0.03 0.004 16.3 0.003 0.033 0.13 Example C 0.036 0.230.68 0.04 0.007 16.2 0.002 0.050 0.52 Cu: 0.4 Example D 0.044 0.22 0.620.01 0.004 16.5 0.003 0.046 0.17 V: 0.03, Ti: 0.01, Nb: 0.02 Example E0.027 0.03 0.77 0.02 0.006 16.4 0.005 0.034 0.17 Example F 0.024 0.480.82 0.03 0.004 16.2 0.004 0.031 0.14 Example G 0.037 0.29 0.03 0.010.003 16.4 0.003 0.045 0.12 Example H 0.036 0.26 0.97 0.02 0.003 16.10.005 0.044 0.18 Example I 0.043 0.16 0.66 0.03 0.004 16.3 0.094 0.0390.15 Example J 0.041 0.25 0.67 0.03 0.004 16.2 0.005 0.048 0.11 Mo: 0.3,Co: 0.2 Example K 0.039 0.24 0.70 0.04 0.006 16.1 0.008 0.052 0.16 Mg:0.001, B: 0.001, Ca: 0.001 Example L 0.036 0.21 0.63 0.04 0.005 16.10.007 0.049 0.14 REM: 0.04 Example M 0.044 0.26 0.61 0.03 0.004 14.90.004 0.040 0.13 Comparative Example N 0.048 0.28 0.63 0.04 0.006 19.30.006 0.043 0.18 Comparative Example •The remainder other than theconstituent chemical elements described above is Fe and inevitableimpurities.

TABLE 2 Ending Starting Starting Starting Ending Accumulated Hot rolledThickness Temper- Thickness Temper- Ending Thickness Rolling sheet Rustof Rough ature of of ature of Temperature of 7th Reduction RatioAnnealing Area Steel Rolling 5th Pass 5th Pass 6th Pass of 7th Pass Passof Final 3 Passes Temperature E_(L) E_(D) E_(C) |ΔE| Planar RatioCorrosion No. Code [mm] [° C.] [mm] [° C.] [° C.] [mm] [%] [° C.][GPa]⁽*¹⁾ [GPa]⁽*²⁾ [GPa]⁽*³⁾ [GPa]⁽*⁴⁾ Anisotropy [%] Resistance Note 1A 40.7 1018 17.8 998 975 13.2 26 Undone 212 192 234 31 ◯ 12 ◯ Example 2A 40.1 1014 17.5 995 972 12.4 29 Undone 214 194 222 24 ◯ 12 ◯ Example 3A 39.9 1017 17.8 997 977 11.6 35 Undone 210 205 220 10 ◯ 20 ◯ Example 4A 40.4 1010 17.8 990 968 10.7 40 Undone 197 206 220  3 ◯ 14 ◯ Example 5A 40.4 1029 8.8 1014 986 6.4 27 Undone 215 194 232 30 ◯ 11 ◯ Example 6 A40.0 1023 11.7 1009 982 8.1 31 Undone 213 198 227 22 ◯ 11 ◯ Example 7 A39.1 1011 7.5 992 967 4.2 44 Undone 201 199 227 15 ◯ 14 ◯ Example 8 A39.3 962 8.8 942 923 6.3 28 Undone 212 201 217 14 ◯ 15 ◯ Example 9 A40.9 1097 10.2 1083 1044 6.2 39 Undone 196 204 234 11 ◯ 12 ◯ Example 10B 40.7 1029 7.9 1009 989 5.9 25 Undone 208 196 230 23 ◯ 19 ◯ Example 11B 39.9 1015 8.8 1001 972 6.0 32 Undone 207 204 226 12 ◯ 12 ◯ Example 12B 40.4 1026 9.2 1013 986 5.6 39 Undone 203 205 227 10 ◯ 14 ◯ Example 13B 39.6 977 12.2 960 920 8.2 33 Undone 193 217 216 13 ◯ 14 ◯ Example 14 C39.9 1078 9.5 1064 1011 6.1 36 Undone 206 203 230 15 ◯ 3 ⊙ Example 15 C40.2 996 9.4 979 945 5.8 38 Undone 217 204 236 23 ◯ 2 ⊙ Example 16 C39.3 1061 8.0 1046 1003 4.3 46 Undone 205 212 233  7 ◯ 2 ⊙ Example 17 C40.3 1069 16.0 1051 1038 11.7 27 Undone 211 196 221 20 ◯ 2 ⊙ Example 18D 39.7 1033 10.6 1019 994 7.4 30 Undone 209 202 224 15 ◯ 16 ◯ Example 19D 39.8 1037 12.3 1021 1000 9.2 25 Undone 204 199 228 17 ◯ 15 ◯ Example20 D 41.0 962 11.8 946 903 8.5 28 Undone 206 200 221 14 ◯ 14 ◯ Example21 D 40.5 949 10.3 930 904 6.0 42 Undone 214 205 226 15 ◯ 11 ◯ Example22 A 40.4 1021 8.2 1002 983 6.4 22 Undone 222 189 234 39 X 18 ◯Comparative Example 23 A 40.7 924 8.3 907 876 5.7 31 Undone 225 186 23042 X 18 ◯ Comparative Example 24 A 39.7 891 9.0 876 852 6.0 33 Undone226 185 236 46 X 18 ◯ Comparative Example 25 B 40.5 1030 17.9 1009 98212.2 32 889 213 191 226 29 ◯ 19 ◯ Example 26 B 40.8 1042 18.4 1013 97712.2 34 806 218 206 232 19 ◯ 15 ◯ Example 27 E 40.4 983 18.0 947 92210.6 41 Undone 201 187 224 26 ◯ 15 ◯ Example 28 F 40.0 974 17.7 951 92910.8 39 Undone 213 192 205 17 ◯ 15 ◯ Example 29 G 40.3 1006 20.1 977 95314.7 27 Undone 206 189 231 30 ◯ 19 ◯ Example 30 H 39.7 1111 20.5 984 95714.9 27 Undone 211 186 229 34 ◯ 15 ◯ Example 31 I 40.6 943 18.8 919 90313.0 31 Undone 219 193 228 31 ◯ 20 ◯ Example 32 J 40.5 966 19.2 932 90913.3 31 Undone 208 199 221 16 ◯ 2 ⊙ Example 33 K 40.5 953 19.4 927 90513.2 32 Undone 206 187 208 20 ◯ 15 ◯ Example 34 L 39.9 962 19.0 931 91013.3 30 Undone 219 207 231 18 ◯ 16 ◯ Example 35 M 40.1 980 18.4 949 91813.5 27 Undone 212 186 226 33 ◯ 38 X Comparative Example 36 N It was notpossible to perform the evaluations due to fracturing in the hot rollingprocess. Comparative Example 37 B 40.2 1152 9.4 1134 1109 6.1 35 Undone222 181 229 45 X 13 ◯ Comparative Example 38 B 40.6 1047 18.6 1018 98612.3 34 936 216 186 235 40 X 13 ◯ Comparative Example ⁽*¹⁾ E_(L):longitudinal elasticity modulus in a direction parallel to the rollingdirection ⁽*²⁾ E_(D): longitudinal elasticity modulus in a direction atan angle of 45° to the rolling direction ⁽*³⁾ E_(C): longitudinalelasticity modulus in a direction at a right angle to the rollingdirection ⁽*⁴⁾ |ΔE| = |(E_(L) − 2 × E_(D) + E_(C))/2| An underlinedportion indicates a value out of the range according to the presentinvention.

Nos. 1 through 21 and Nos. 25 through 34, which satisfied therequirements according to aspects of the present invention regarding theranges of a chemical composition, hot rolling conditions, and hot rolledsheet annealing conditions, had a small absolute value (|ΔE|) of planaranisotropy in terms of the modulus of longitudinal elasticity of 35 GPaor less, which means these examples had the desired rigidity. Moreover,from the results of the evaluation of corrosion resistance performed onthe obtained hot rolled steel sheets and hot rolled and annealed steelsheets, it is clarified that all the steel sheets had a rust area ratioof 25% or less, which means these examples also had sufficient corrosionresistance.

In particular, Nos. 14 through 17 manufactured by using steel C, whichcontained 0.52 mass % of Ni and 0.4 mass % of Cu, and No. 32manufactured by using steel J, which contained 0.3 mass % of Mo, had ahigher level of excellent corrosion resistance represented by a rustarea ratio of 10% or less.

In the case of No. 22 where the accumulated rolling reduction ratio ofthe final 3 passes was less than the range according to aspects of thepresent invention, since a large number of elongated grains existed inthe central portion in the thickness direction, there was an increase inthe degree of planar anisotropy in terms of modulus of longitudinalelasticity so that it was not possible to achieve the specified |ΔE|.

In the case of No. 23 where only the final temperature of the seventhpass among the rolling in the final 3 passes is lower than the rangeaccording to aspects of the present invention and in the case of No. 24where all the rolling temperatures of the final 3 passes were lower thanthe range according to aspects of the present invention, since there wasinsufficient recrystallization in the central portion in the thicknessdirection even though rolling was performed with the specifiedaccumulated rolling reduction ratio, it was not possible to achieve thespecified |ΔE|. In addition, in the case of No. 37 where all the rollingtemperatures of the final 3 passes were higher than the range accordingto aspects of the present invention, since there was an increase incrystal grain size, it was not possible to achieve the specified |ΔE|.

In the case of No. 38 where the hot rolled sheet annealing temperaturewas higher than the range according to aspects of the present invention,since austenite was formed when hot rolled sheet annealing wasperformed, it was not possible to achieve the specified |ΔE|. In thecase where the steel sheets of Nos. 22 through 24, 37, and 38, in whichit was not possible to achieve the specified |ΔE|, were used for flangeshaving a large wall thickness, it was clarified that bending and twistoccurred when the flanges were subjected to vibration.

In the case of No. 35 manufactured by using steel M, whose Cr contentwas less than the range according to aspects of the present invention,since there was an insufficient amount of passivation film formed on thesurface of the steel sheet, it was not possible to achieve the desiredcorrosion resistance.

In the case of No. 36 manufactured by using steel N, whose Cr contentwas over the range according to aspects of the present invention, sincefracturing occurred during the hot rolling process due to a crack formedin the slab when cooling was performed after casting had been performed,it was not possible to perform the specified evaluations.

INDUSTRIAL APPLICABILITY

The hot rolled ferritic stainless steel sheet obtained by using aspectsof the present invention can particularly preferably be used forpurposes which require satisfactory rigidity and corrosion resistance,for example, for the flange of an EGR cooler.

1. A hot rolled ferritic stainless steel sheet having a chemicalcomposition containing, by mass %, C: 0.005% to 0.060%, Si: 0.02% to0.50%, Mn: 0.01% to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5%to 18.0%, Al: 0.001% to 0.10%, N: 0.005% to 0.100%, Ni: 0.1% to 1.0%,and the balance being Fe and inevitable impurities and an absolute value|ΔE| of planar anisotropy in terms of modulus of longitudinal elasticitycalculated by using equation (1) below of 35 GPa or less:|ΔE|=|(E _(L)−2×E _(D) +E _(C))/2|  (1), where, E_(L) denotes modulus oflongitudinal elasticity (GPa) in a direction parallel to the rollingdirection, E_(D) denotes modulus of longitudinal elasticity (GPa) in adirection at an angle of 45° to the rolling direction, and E_(C) denotesmodulus of longitudinal elasticity (GPa) in a direction at a right angleto the rolling direction.
 2. The hot rolled ferritic stainless steelsheet according to claim 1, the steel sheet having the chemicalcomposition further containing, by mass %, one, two, or more selectedfrom Cu: 0.1% to 1.0%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.5%.
 3. Thehot rolled ferritic stainless steel sheet according to claim 1, thesteel sheet having the chemical composition further containing, by mass%, one, two, or more selected from V: 0.01% to 0.25%, Ti: 0.001% to0.015%, Nb: 0.001% to 0.025%, Mg: 0.0002% to 0.0050%, B: 0.0002% to0.0050%, Ca: 0.0002% to 0.0020%, and REM: 0.01% to 0.10%.
 4. The hotrolled ferritic stainless steel sheet according to claim 2, the steelsheet having the chemical composition further containing, by mass %,one, two or more selected from V: 0.01% to 0.25%, Ti: 0.001% to 0.015%Nb: 0.001% to 0.025%, Mg: 0.0002% to 0.0050%, B: 0.0002% to 0.0050% Ca:0.0002% to 0.0020%, and REM: 0.01% to 0.10%.
 5. A hot rolled andannealed ferritic stainless steel sheet obtained by performing hotrolled sheet annealing on the hot rolled ferritic stainless steel sheetaccording to claim
 1. 6. A hot rolled and annealed ferritic stainlesssteel sheet obtained by performing hot rolled sheet annealing on the hotrolled ferritic stainless steel sheet according to claim
 2. 7. A hotrolled and annealed ferritic stainless steel sheet obtained byperforming hot rolled sheet annealing on the hot rolled ferriticstainless steel sheet according to claim
 3. 8. A hot rolled and annealedferritic stainless steel sheet obtained by performing hot rolled sheetannealing on the hot rolled ferritic stainless steel sheet according toclaim
 4. 9. A method for manufacturing the hot rolled ferritic stainlesssteel sheet according to claim 1, the method comprising performing a hotrolling process involving finish rolling composed of 3 passes or more,wherein rolling in the final 3 passes of the finish rolling is performedin a temperature range of 900° C. to 1100° C. with an accumulatedrolling reduction ratio of 25% or more.
 10. A method for manufacturingthe hot rolled ferritic stainless steel sheet according to claim 2, themethod comprising performing a hot rolling process involving finishrolling composed of 3 passes or more, wherein rolling in the final 3passes of the finish rolling is performed in a temperature range of 900°C. to 1100° C. with an accumulated rolling reduction ratio of 25% ormore.
 11. A method for manufacturing the hot rolled ferritic stainlesssteel sheet according to claim 3, the method comprising performing a hotrolling process involving finish rolling composed of 3 passes or more,wherein rolling in the final 3 passes of the finish rolling is performedin a temperature range of 900° C. to 1100° C. with an accumulatedrolling reduction ratio of 25% or more.
 12. A method for manufacturingthe hot rolled ferritic stainless steel sheet according to claim 4, themethod comprising performing a hot rolling process involving finishrolling composed of 3 passes or more, wherein rolling in the final 3passes of the finish rolling is performed in a temperature range of 900°C. to 1100° C. with an accumulated rolling reduction ratio of 25% ormore.
 13. A method for manufacturing a hot rolled and annealed ferriticstainless steel sheet, the method comprising using the method formanufacturing a hot rolled ferritic stainless steel sheet according toclaim 9, and further performing hot rolled sheet annealing at atemperature of 800° C. to 900° C. after the hot rolling process.
 14. Amethod for manufacturing a hot rolled and annealed ferritic stainlesssteel sheet, the method comprising using the method for manufacturing ahot rolled ferritic stainless steel sheet according to claim 10, andfurther performing hot rolled sheet annealing at a temperature of 800°C. to 900° C. after the hot rolling process.
 15. A method formanufacturing a hot rolled and annealed ferritic stainless steel sheet,the method comprising using the method for manufacturing a hot rolledferritic stainless steel sheet according to claim 11, and furtherperforming hot rolled sheet annealing at a temperature of 800° C. to900° C. after the hot rolling process.
 16. A method for manufacturing ahot rolled and annealed ferritic stainless steel sheet, the methodcomprising using the method for manufacturing a hot rolled ferriticstainless steel sheet according to claim 12, and further performing hotrolled sheet annealing at a temperature of 800° C. to 900° C. after thehot rolling process.